Laser modification of healthy dentin above the physical modification threshold increases microhardness, resistance to acid attack and alters the mineral and organic constituents of dentin. In the first project period, the optical properties of healthy dentin in the ultraviolet, visible and infrared regions of the electromagnetic spectrum were defined. The optical properties and physical modification threshold of coronal caries are under investigation, but the optical properties and physical modification threshold of coronal transparent and sclerotic root dentin have yet to be defined. The importance of this work is that the understanding of the optical properties and physical modification threshold of these forms of dentin may be beneficial in the development of diagnostic, selective ablation and modification techniques. Additionally, understanding the minimum laser parameters required to modify these forms of dentin will lead to the safest laser treatment. Current research has focused on laser interactions at or above the physical modification threshold of dentin. With these systems, the surface changes occur very close to the ablation threshold. Since in the clinical setting, dentists are removing gross caries and are bonding to transparent dentin coronally, and bonding to sclerotic dentin with no treatment, these forms of dentin require careful study to determine the interaction of lasers with them. Other forms of dentin, such as secondary and reparative are typically not encountered in the clinical situation, although interesting will not yield clinically significant findings. The utilization of pulsed fiberoptic short pulsed lasers on various forms of dentin below the physical modification threshold may produce alterations in the collagen structure of dentin which will increase availability of free radicals for chemical bonding. This project will also initiate the development, as a pilot project, of a restorative material that when exposed to laser energy will melt and fuse to dentin. The specific hypotheses to be tested are that pulsed fiber optic delivered lasers used on coronal transparent and sclerotic root dentin significantly modify the dentin to increase microhardness, resistance to acid attack with selective ablation (compared to healthy dentin) and that wetting of adhesive resins will be increased with increased bond strength. Changes induced in collagen below the physical modification of dentin may produce free radicals that will be available for bonding to composite resins. Photon sensitive polymers will be developed that will fuse to dentin after exposure to laser light.